Final answer:
The technique that doesn't belong to requirements techniques for an existing system is Design class diagram. Class diagrams are part of the design phase, not the requirements gathering, which focuses on understanding the existing system and defining the new system's needs.
Step-by-step explanation:
The question refers to the techniques used when there is an existing system that one wants to replace or enhance. Among the options listed, Design ERD (Entity Relationship Diagram), Design class diagram, Build prototypes, and Create a feature tree, the one that does not belong to the requirements techniques would be Design class diagram. The reason is that class diagrams are more related to object-oriented design rather than gathering requirements for an existing system. While ERD might be used to understand the data relationships in the existing system, class diagrams are part of the design phase which follows the requirements gathering.
When identifying a problem or a need, the engineers should focus on the attributes of the product, which involves understanding what the existing system does and documenting the requirements for the new or enhanced system. Build prototypes is a crucial step in testing the concepts of the new requirements. Prototypes help in the evaluation and refinement of the design before final implementation, even if sometimes they are not tested adequately. Lastly, creating a feature tree helps to organize and prioritize features, which forms part of requirements engineering as opposed to the actual designing of the system architecture which involves creating class diagrams.